In accordance with a known method, the two ends of the rod are successively embedded in their end fittings and then the rigid assembly thus obtained is disposed in a moulding apparatus for casting the insulating covering thereover. In outline the molding apparatus has two end parts which surround the end fittings and between which two half shells are inserted which are shaped to match the covering to be formed. For proper connection of the covering to the edges of the end fittings, it is essential for the various component parts of the moulding apparatus to be positioned very accurately to these end fittings: in particular, this requires the distance between facing surfaces of the end fittings to be determined accurately once the two ends are embedded. Said requirement entails precise embedding operations.
Further, at the time of casting, the temperature of the moulding apparatus is a few hundreds of degrees centigrade depending on the insulator used, e.g. 200.degree. C.; the parts of the steel mould, the end fittings generally made of aluminium or the like, and the fibre glass rod have different coefficients of expansion; this results in stresses on the rod and on the end fittings, which stresses can subsequently reduce the fixing power of the end fittings.
Lastly, the substance for embedding the rod is in danger of being damaged due to the fact that it is subjected to the high temperature of the mould.
Preferred implementations of the present invention remedy the above drawbacks.